Brings sbt-frees-protogen as a separate Artifact

[juanpedromoreno]

Towards fixing of #25, this PR separates the protocol buffer files generation from the frees-rpc core.

[fedefernandez]

Code looks good

[codecov-io]

Codecov Report

Merging #49 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   12.01%   11.92%   -0.09%     
==========================================
  Files          20       21       +1     
  Lines         283      285       +2     
  Branches        2        1       -1     
==========================================
  Hits           34       34              
- Misses        249      251       +2

Impacted Files	Coverage Δ
protogen/src/main/scala/proto/converters.scala	0% <ø> (ø)
rpc/src/main/scala/proto/annotations.scala	0% <ø> (ø)	⬆️
protogen/src/main/scala/ProtoCodeGen.scala	0% <ø> (ø)
protogen/src/main/scala/proto/encoders.scala	0% <ø> (ø)
protogen/src/main/scala/proto/processors.scala	0% <ø> (ø)
protogen/src/main/scala/ProtoGenPlugin.scala	0% <0%> (ø)

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